United States Patent Application, Ser. No. 956,955, filed Nov. 2, 1978 by the same inventors and assigned to the same assignee as this application, and hereby incorporated by reference, describes a synthesized local oscillator for a television receiver. The synthesizer reference oscillator described therein is selectively capable of operating in either a voltage-controlled mode whereby its operating frequency is determined by the voltage-applied to its tuning voltage control terminal or in an injection-locked mode whereby the reference oscillator is coupled to a crystal oscillator and its operating frequency is rendered equal to the frequency of the crystal oscillator.
Voltage-controlled operation of the reference oscillator is desirable when the receiver is operating from sources, such as some MATV and CATV systems as well as home video games, that provide nonstandard carrier signal frequencies. In this mode of operation the tuning voltage may be provided either by the AFC circuitry conventionally included in the receiver or by a manually variable FINE TUNE circuit. In either event the reference oscillator operating frequency will be continuously variable so as to accommodate the particular carrier frequency encountered. However, crystal-controlled operation of the reference oscillator is desirable when receiving carrier signals at the FCC-specified frequencies. In addition, even when operating in the voltage-controlled mode, it is desirable to maintain momentary crystal-controlled operation while the synthesizer phase-locked loop (PLL) is undergoing loop acquisition. This may occur when, for example, the viewer makes a channel selection or changes the source of tuning voltage from the FINE TUNE circuit to the AFC circuitry. In the manner described hereinafter, the subject invention comprises a control circuit for selectively effecting reference oscillator operation in the appropriate, that is, either voltage-controlled or injection-locked, mode.